A receiver may be configured to receive and process signals that have broad bandwidth spectra and powers within a certain, expected, range. For example, a receiver on a satellite may be configured to receive a group of signals that share a common region of the electromagnetic spectrum, and are multiplexed with one another using techniques known in the art. In the multiplexing technique known as code division multiple access (CDMA), each signal of the group is encoded with a unique code, and spread over the same selected portion of the spectrum as the other signals in the group. The receiver receives the group of signals, and then decodes one or more of the signals from others in the group using a priori knowledge about the unique code(s) of those signals. Alternatively, in the multiplexing technique known as frequency-division multiple access (FDMA), each signal of the group is assigned a different sub-portion of the region of the spectrum than the other signals in the group. The receiver receives and processes the group of signals, and then differentiates one or more of the signals from others in the group using a priori knowledge about the spectral sub-portion(s) of those signals. The groups of signals received in both CDMA and FDMA may be considered “broad bandwidth” signals because the groups of signals occupy a portion of the electromagnetic spectrum that is significantly broader than normally would be used for a single, non-multiplexed signal, that is, a “narrow bandwidth” signal.
In both CDMA and FDMA, the overall power of the group of signals received by the receiver preferably is sufficiently higher than any noise sources that may be present to yield a sufficient signal-to-noise ratio (SNR) to communicate signals with adequate fidelity as measured by BER (Bit Error Rate) values. At the same time, the overall power of the group of signals also preferably is sufficiently low that the receiver may process the signals without distortion. Specifically, as is known in the art, receivers have a linear range of operation and a nonlinear range of operation. If a signal input to the receiver has a power that falls within the linear range of the receiver, then the receiver processes the received signal collection without distortion. However, if a signal input to the receiver has a power that falls within the nonlinear range of the receiver, then the received signal collection is distorted and communication performance is degraded.
Signals other than the desired group of signals that the receiver receives may be referred to as “interference,” may be intentional or unintentional, may be directional or nondirectional, and may have a broad bandwidth or a narrow bandwidth. If the receiver receives interference that falls within the same portion of the electromagnetic spectrum as the desired group of signals, then the receiver may not distinguish the interference from the group of signals again degrading communication performance. However, if the power of the interference is sufficiently high that nonlinear receiver operation occurs, not only may the interference obscure desired spectral components but also cause additional signal distortion. This additional receiver distortion may include suppression of desired signals and generation of intermodulation products between design signal components and the interference, resulting in additional degradation in receiver performance.
A receiver may have features intended to reduce the effects of such interference. For example, the receiver may be designed so as to increase its linear range, and thus reduce the risk that interference may cause distortion, e.g., by providing circuitry that remains linear at higher input power levels. However, increasing the linear range of the receiver may be expensive, and also may require a larger power supply to operate the modified circuitry.
Another known approach for reducing the effect of narrow bandwidth interference on reception of a broad bandwidth desired signal uses adaptive notch filter techniques. Specifically, a notch filter may be applied to the received signal prior to amplification so as to block the region of the spectrum where the interference is located. The amplitude, width, and spectral location of the notch filter may be adaptively modified over time by varying weighting coefficients, which may be iteratively derived using a gradient process based on an optimization criterion, such as maximum signal to noise plus interference ratio (SNIR). Such adaptive notch filter techniques have been widely applied. However, its iterative nature makes this approach is relatively slow, and thus less able to respond to rapidly changing interference.
The CDMA signal format is an example of spread spectrum modulation wherein user signals are spread over a much wider bandwidth than needed to convey the information in the user's signal. One advantage of spread spectrum modulation is protection from interference achieved by processing the user-unique codes. Similar interference protection may be achieved in FDMA formats by frequency hopping the user assigned frequency slots over a wide bandwidth in a pseudorandom sequence of frequency hop codes known to both the sender and receiver. Signal error correcting coding and interleaving techniques further add to the interference protection and are commonly used. These interference protection techniques are known in the art, but their benefits depend on linear receiver operation. The effectiveness of these techniques is significantly degraded by receiver nonlinearities.
In addition to negating interference in the spectral domain, e.g., filtering narrow bandwidth interference, interference may also be spatially negated. This spatial processing, commonly referred to as adaptive antenna interference cancellation, combines two or more antenna elements to produce pattern null(s) in the direction of the interference source(s). Such pattern nulls likewise reduce the received interference power. Conventional adaptive antennas use iterative algorithms to establish pattern nulls and like adaptive notch filters, and accordingly cannot sufficiently respond to rapidly changing interference environments.
Thus, what is needed is a method of reducing the effects of interference with broad bandwidth signals while maintaining linear receiver operation.